Whatever and no matter how much is thrown into the pit, it stays empty. Is there a better substitute for "no matter how much" in the above sentence? like the word 'whatever' is being used for 'no matter what', what single word can be used to indicate 'any quantity' here?
No matter how much.
However much it might embarrass you, you're taking your little sister to that concert!
In this case, you could say:
Whatever and however much is thrown into the pit, it stays empty.
A single adjective which describes that is bottomless. The Oxford Dictionaries has this
A pit whose bottom cannot be seen or found; an abyss, hell; (in extended use) something which can never be filled or satisfied, or which is inexhaustible or unfathomable.
The phrase is often used figuratively.